King Arthur's knights use a catapult to launch a rock from their vantage point on top of the castle wall, 11 above the moat. The rock is launched at a speed of 29 m/s and an angle of 40 above the horizontal. How far does the rock land from the castle? I wanted to add that I see a question similar to this one but I do not understand where they got the equation they used, also someone commented that it was incorrect. I really need some assistance here, this is due in a matter of hours, and I have been stuck on this stupid problem for days.

Question

King Arthur's knights use a catapult to launch a rock from their vantage point on top of the castle wall, 11  above the moat. The rock is launched at a speed of 29 m/s and an angle of 40 above the horizontal. How far does the rock land from the castle? I wanted to add that I see a question similar to this one but I do not understand where they got the equation they used, also someone commented that it was incorrect. I really need some assistance here, this is due in a matter of hours, and I have been stuck on this stupid problem for days.